The invention relates generally to electronic fuel control systems for compression ignition engines and, more particularly, to a fuel injection control system that suppresses emission generation of compression ignition diesel engines.
Diesel engines are well known for producing black smoke or heavy particulate emissions during acceleration or load ascending transients. One cause of this phenomenon is the late burning associated with the combustion of fuel injected in compression cylinders during these acceleration and load ascending transient engine operating modes.
The basic combustion process for diesel engines involves a diffusion type combustion of liquid fuel. As liquid fuel is injected into compressed hot cylinder air, it evaporates and mixes with the surrounding air to form a flammable mixture. This is a continuing process that happens over time as the fuel is injected into the cylinder. The mixture formed initially will combust and raise the local temperature before the later evaporated fuel has time to fully mix with air. As a result, the later burned fuel is subjected to high temperatures with insufficient air. Under such conditions, high temperature pyrolysis of fuel will take place and thus form soot. As the combustion proceeds in the cylinder, a substantial portion of this soot will be burned-up as a result of later exposure to available air in the cylinder. The soot will continue to be burned up in the engine until the power stroke volume expansion sufficiently lowers the cylinder temperature, thereby ceasing the chemical reaction. Any non-combusted soot remaining in the cylinder at this point exits the engine as smoke or particulate emission when the exhaust valve is opened.
In compression combustion engines, therefore, two opposing mechanisms for soot occurrence exist: soot formation and soot burn-up. In typical combustion engines under typical operating conditions the soot burn-up mechanism is sufficient to reduce emissions caused by soot formation. However, in certain engines operating under accelerating or load ascending transient conditions, the soot burn-up mechanism is insufficient for reducing the generation of soot emissions, as is discussed more fully herein below. Late burning of injected fuel results in engines operating under acceleration or load ascending transient conditions. As such, adequate time is not provided for the occurrence of the soot burn-up process prior to opening of the exhaust valve. Thus, the significant expulsion of smoke and particulate emission is common in a large diesel engine operating under accelerating or load ascending transient conditions.
Compression ignition engines of the prior art typically have fixed injection timing via a governor and mechanical linkages which actuate a series of fuel delivery devices simultaneously. Fuel injection start timing is generally predetermined for any given engine operating point and typically cannot be modified for varying conditions. Fuel delivery systems may include pump-line-nozzle configurations or unit injection configurations. An electronic fuel injection system for large cylinder volume displacement diesel engines is disclosed in U.S. Pat. No. 5,394,851. This prior art fuel injection system is employed in conjunction with a typical compression ignition diesel engine shown generally at 10 in FIG. 1. The engine 10 may be any large diesel engine. Such an engine may include a turbo charger 12 and a series of unitized power assemblies 14. For example, a twelve-cylinder engine has twelve such power assemblies while a sixteen-cylinder engine has sixteen such power assemblies. The engine 10 further includes an air intake manifold 16, a fuel supply line 18 for supplying fuel to each of the power assemblies 14, a water inlet manifold 20 used in cooling the engine, a lube oil pump 22 and a water pump 24, all as known in the art. An intercooler 26 connected to the turbo charger 12 facilitates cooling of the turbo charged air before it enters a respective combustion chamber inside one of the power assemblies 14. The engine may be a Vee-style type, also as known in the art.
Although well suited for its application, the system of FIG. 1 neither distinguishes nor does it accommodate for accelerating and load ascending transient operating modes and the effect of these operating modes upon the generation of emissions due to late combustion as discussed herein. In such systems, the fuel injection timing of a diesel engine is usually prescribed for each operating condition (speed and load) at its optimum for steady state operation. When the engine is experiencing load ascending transients or acceleration, the injection timing will still be set at its instantaneous value called for by the steady state condition. Operating under a steady state condition, there is usually enough time in the combustion cylinder to control particulate or smoke emissions via the soot burn-up process described herein above. During load ascending or acceleration transients, however, the engine calls for more fuel thus the fuel injection duration becomes longer. The combustion of the added fuel, which enters the cylinder at the end of the injection duration, does not have enough time for soot burn-up before the exhaust valve opens. The result is the increased emission of heavy smoke or particulate matter during the exhaust stage of the engine cycle. This is particularly true for the modern-day low emission diesel engine, which applies retarded fuel injection timing during steady state operation in the attempt to reduce NOx emissions.
Normal acceleration of a diesel engine (such as a medium speed engine for locomotive applications) produces transient conditions which vary from steady state conditions and increase the production of soot and particulate emissions. Such engines also encounter radical load changes due to the switching of large auxiliary loads such as compressor loads or fan loads in locomotive applications and "hotel" power loads (an alternator for generating 110 V at 60 hz) for passenger train applications. Driving such loads or turning off such loads can result in load transients on the order of 500 horsepower at any instant. Late burning of injected fuel, as discussed herein above, is prevalent in such acceleration and load ascending transient diesel engine operating modes. The late burning prevents proper combustion of generated soot and results in increased engine expulsion of smoke and particulate emissions.
Therefore, it is desirable to suppress the smoke expulsion and particulate emission during acceleration and load ascending transient operating modes of a compression ignition engine and also maintain proper operation during steady state modes.